Wireless sensor networks are known to be used in process control systems. Examples on such networks are ZigBee and WirelessHart, ISA100, WIA-PA and Bluetooth. There also exist some WLAN based sensor networks.
These networks typically employ a time division communication scheme where a wireless communication device in the form of field device may access resources in the form of time intervals of the scheme for communication with the process control system.
Some wireless networks use a fixed allocation of the time interval to field devices. This requires a network manager or radio resource manager that manages the resources.
In order to speed up and also make the allocation of resources more flexible, it is of interest to use contention based access. In this case field devices compete for the resources and determine which field device is to occupy or seize a resource based on arbitration. However, this has been somewhat hard to implement in wireless systems because of the difficulties in implementing simultaneous transmission and reception.
There has been suggested some different schemes for providing contention based access in a wireless network, i.e. a field device accesses a time interval based on arbitration with competing field devices.
The article “Recent and Emerging Topics in Wireless Industrial Communications: A Selection”, by Andreas Willig, IEEE Transactions on Industrial Informatics, Vol. 4, No. 2, page 102-124 mentions a number of approaches through which contention based access can be provided.
One approach mentioned is that all field devices transmit a jamming signal, where the length of the jamming signal indicates the priority. After having transmitted a jamming signal a field device switches to receiving and defers from transmitting if a jamming signal is then received and otherwise transmits data. In this way it is ensured that the field device having the highest priority will access the resource.
Another mentioned approach is to let the field devices listen on a channel for a time proportional to these priorities, where a high priority involves a short listening period. A field device then starts to transmit at the end of the interval if no other field device has already started to transmit.
Yet another approach mentioned is to have a bit-wise priority scheme through providing one time slot for each priority bit. The field devices will then transmit in the interval of the priority, where the highest priority is first. As soon as a field device with a lower priority receives a signal from a field device of higher priority, it gives up.
The latter scheme is also described in US 2009/0310571.
There is a problem with all these schemes and that is that there is a lot of time and effort needed for performing the arbitration. This time period is furthermore more or less dependent on the number of priority levels. This problem is therefore getting severe if there are many different priority levels.
There is therefore a need for an improvement in relation to contention based access of resources in a wireless network.